The present invention relates to a metal powder granulate comprising one or more of the metals Co, Cu, Ni, W and Mo, a process for its preparation and its use.
Granulates of the metals Co, Cu, Ni, W and Mo have many applications as sintered materials. For example copper metal granulates are suitable for preparing copper sliding contacts for motors, tungsten granulates can be used to prepare W/Cu infiltration contacts, Ni and Mo granulates may be used for corresponding semi-finished applications. Cobalt metal powder granulates are used as binder components in composite sintered items, e.g. hard metals and diamond tools.
DE-A 43 43 594 discloses that free-flowing metal powder granulates can be prepared by pulverising and screening out a suitable range of particle sizes. However, these granulates are not suitable for producing diamond tools.
EP-A-399 375 describes the preparation of a free-flowing tungsten carbide/cobalt metal powder granulate. As starting components, the fine powders are agglomerated, together with a binder and a solvent. In a further process step the binder is then removed thermally and the agglomerate is after-treated at 2500.degree. C. in a plasma in order to obtain the desired free-flowing property. Fine cobalt metal powder, however, cannot be granulated using this process because similar processing problems occur at temperatures above the melting point as those encountered during the processing of very fine powders.
DE-A 44 31 723 discloses that pastes of oxide compounds can be obtained if water-dilutable, non-ionogenic rheological additives are added. These additives may be thermally removed, resulting in compact layers on substrates. However, the objective of this process is to coat the substrate with finely divided, completely agglomerate-free particles.
EP-A 0 659 508 describes the preparation of metal powder granulates of the general formula RFeB and RCo, wherein R represents rare-earth metals or compounds, B represents boron and Fe represents iron. Here, an alloy of the components is first prepared and this is reduced to the desired fineness by milling. Then binder and solvent are added and the slurry is dried in a spray drier. The disadvantage of this process, in particular for preparing diamond tools, is that the metals are first alloyed and the fine cobalt powders lose their characteristic properties due to the melting procedure, as described in DE-A 43 43 594. The prior art for producing cobalt metal powder granulates is therefore to add binders or organic solvents to fine cobalt metal powder and to produce corresponding granulates in suitable granulating devices, as can be deduced e.g. from the brochures relating to the granulating machine G10 from the Dr. Fritsch KG Co., Fellbach in Germany and for the solids processor from the PK-Niro Co. in Soeberg, Denmark. The solvents are carefully removed after granulation by an evaporation procedure, but the binder remains in the granulates and has a significant effect on the properties.
The granular particles obtained in this way have a rounded shape. The surface is relatively compact without large pores or openings for the escape of gases. The bulk density determined in accordance with ASTM B 329 is relatively high, 2.0 to 2.4 g/cm.sup.3 (Table 2). FIG. 1 shows the scanning electron (SEM) photograph of a commercially available granulate from the Eurotungstene Co., Grenoble, France, and FIG. 2 shows a commercially available granular material from the Hoboken Co., Overpelt, Belgium. Although the rounded shape of the particles and the high bulk densities lead to the desired improved flow properties for cobalt, processing problems are still not inconsiderable in practice.
For example, relatively high compression forces have to applied during cold compression in order to obtain preforms with sufficient strength and edge stability. The reason for this is that the production of firmly interlocking compounds, i.e. expressed more simply, the hooking together of the individual particles, which is important for providing strength in the preforms, is difficult with spherical or rounded particles. At the same time, a dense, closed structure leads to an increase in the resistance to deformation. Both factors lead to an increase in the compression forces required during cold compression. This can in practice, however, cause increasing wear on the cold compression moulds, i.e. to lower durability of the cold compression moulds, which again leads to increased production costs.
Quantitatively, the compression behaviour can be described by measuring the compaction factor F.sub.comp. F.sub.comp is defined by the equation: EQU F.sub.comp =(.rho..sub.p -.rho..sub.o)/.rho..sub.p
where .rho..sub.o is the bulk density in g/cm.sup.3 of the cobalt metal powder granulate in the original state and .rho..sub.p is the density in g/cm.sup.3 after compression.
The most serious disadvantage, however, is that the binder used during preparation of the granulates remains in the granulates (see Table 1).
In the following a binder is understood to mean a film-forming substance which is optionally dissolved in a solvent and added to the starting components in a suitable granulating process so that the powder surface is wetted and, optionally after removing the solvent, holds this together by forming a surface film on the primary particles. Granulates with sufficient mechanical strength are produced in this way. Alternatively, substances which use capillary forces to provide mechanical strength in the granulate particles may also be considered as binders.
TABLE 1 ______________________________________ Typical concentrations of carbon from the binder in commercially available cobalt metal powder granulates. HOBOKEN EUROTUNGSTENE Overpelt, HOBOKEN Grenoble, France Belgium Overpelt, Belgium ______________________________________ Product Co ultrafine Co extrafine Co extrafine granulated soft granulate hard granulate Carbon ca. 1.5% ca. 0.98% ca. 0.96% content ______________________________________
If items are prepared from these cobalt metal powder granulates, for example using the hot compression technique which is most frequently applied, then the heating time must be extended in order to remove the organic binder completely. This may result in a production loss of up to 25%. If, on the other hand, the heating times are not extended, then carbon clusters are observed in the hot compressed segments, these resulting from cracking of the binder. This frequently leads to an obvious impairment in the quality of tools.
A further disadvantage is the use of organic solvents which have to be carefully removed by evaporation after granulation. Firstly, removing the solvent by a thermal process is cost intensive. In addition the use of organic solvents incurs substantial disadvantages with respect to environmental impact, plant safety and the energy balance. The use of organic solvents frequently requires a considerable amount of equipment such as gas extraction and waste treatment devices as well as filters in order to prevent the emission of organic solvents during granulation. A further disadvantage is that the plants have to be protected against explosions, which again increases the production costs.
The disadvantages of working with organic solvents can in theory be avoided by dissolving the binder in water. However, the fine cobalt metal powders are then partially oxidised and therefore cannot be used.
Now, the object of this invention is to provide a metal powder granulate which does not have the disadvantages of the powders described above.